RE SEARCH New Erwinia variants require more knowledge and specific tests For quite some time now, we’ve known two types of Erwinia bacteria, namely Erwinia carotovora subsp. atroseptica and Erwinia chrysanthemi. They are/were responsible for the Black leg and Stem wet rot potato diseases. However, the names of these pathogenic bacteria have been changed. Furthermore, Erwinia variants that didn’t exist or hardly existed before have crept into the cultivation of seed potatoes and can also cause bacterial infections. The question now is what happens when these bacterial diseases occur and what is the importance of research into the diversity of these rot bacteria in order to control them in the cultivation of potatoes. U p to now, field inspections have always determined the inspection results of seed. And it didn’t really matter who decided what. With a laboratory test for Erwinia it can now accurately be established whether planting stock is free from disease or not. In order to be able to apply this laboratory test properly, it must be able to detect every single Erwinia variant. What is needed, therefore is up-to-date knowledge about the diversity of these pathogens. Two Erwinia species Taxonomists have divided Erwinia bacteria that can cause rot into two species: Pectobacterium and Dickeya. Within the Pectobacterium species we know the subsp. P. atrosepticum. This used to be Erwinia carotovora subsp. atroseptica. This species no longer occurs much in the Netherlands. Twenty years ago, however, this pathogen was still responsible for about one third of all bacterial diseases in plants. It’s not quite clear why this bacterium has disappeared. From all the Erwinias, this pathogen thrives best in cold weather. It’s possible that climate change has played a role in its disappearance. The second species of Pectobacterium we can distinguish is Pectobacterium wasabiae. This has also been present in the Netherlands for a long time, but has never been recognised as a species in its own right. Pectobacterium wasabiae is more often found in seed these days. Fortunately, the bacterium is not very agressive and doesn’t cause blackleg as often as other Pectobacterium and Dickeya species. Newcomer can spread fast A newcomer is Pectobacterium carotovorum subsp. brasiliense. This bacterium has now been found on potatoes in the Netherlands for some time, and is already responsible for 20 to 30 percent of all disease incidence. The bacterium was first found in potatoes in Brazil, but has now also been isolated from diseased plants in Africa, New Zealand, North America and various European countries. Research in Wageningen shows that all isolates in the Netherlands are genetically virtually identical. This indicates that the bacterium has spread very quickly all over the Netherlands from a single source. It probaThere are also Erwinia variants that didn’t exist or hardly existed before that have crept into the cultivation of seed potatoes and can also cause bacterial infections. Potato World 2015 • number 4 23 Pagina 22
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